Centrifugal pumps use centrifugal force to move liquids from a lower pressure to a higher pressure and employ an impeller, typically comprising of a connecting hub with a number of vanes and shrouds, rotating in a volute or casing. Liquid drawn into the center of the impeller is accelerated outwardly by the rotating impeller vanes toward the periphery of the casing, where it is then discharged at a higher pressure.
Centrifugal pumps, such as trash pumps, are conventionally used in applications involving mixtures of solids and liquids, solids-laden mixtures, slurries, sludge, raw unscreened sewage, miscellaneous liquids and contaminated trashy fluids, collectively referred to as mixed-media flow or mixed-media fluids. These mixed-media fluids are encountered in applications including, but not limited to, sewage plants, sewage handling applications, paper mills, reduction plants, steel mills, food processing plants, automotive factories, tanneries, and wineries.
As one example, such pumps are used in sewage lift stations to move wastewater to a wastewater treatment plant. In some aspects, submersible pumps are disposed in a wet well below ground (e.g., 20′ below ground) and are configured to lift the wastewater to an elevation just below ground level, where it is passed to downwardly sloping conduits that utilize gravity to move the flow along the conduit to the next lift station. This operation is repeated at subsequent lift stations to move the wastewater to a wastewater treatment plant. Another form of lift station utilizes “dry well” pumps, wherein one or more self-priming centrifugal pumps and associated controls and drivers (i.e., motor or engine) are either located in a (dry) building above ground or in a (dry) fiberglass (or concrete, metal, and/or polymer) room disposed below ground. Above-ground configurations utilize a self-priming centrifugal pump and an intake extending down into a wet well holding the influent wastewater. An exemplary solids-handling self-priming centrifugal pump for such application includes the Gorman Rupp T-Series™ or Super T-Series™ pumps, which feature a large volute design allowing automatic re-priming in a completely open system without the need for suction or discharge check valves and with a partially liquid-filled volute housing and a dry suction line. Depending on the size and configuration, these pumps generally handle a maximum solids diameter of between about 1.5″-3″ with a maximum head of between about 110 ft.-150 ft. Below-ground configurations typically use either a non-self-priming centrifugal pump disposed beneath the wet well, so as to provide a flooded pump suction, or use a self-priming pump. Flooded non-self-priming pumps correspondingly require an isolation means (e.g., a valve) to permit isolation of the pump suction to allow for pump cleaning and maintenance.
The nature of the conveyed medium poses significant challenges to continuous operation of the pumps. One potential problem in such applications is the clogging of the impeller or pump by debris in the pumped medium. Therefore, pump serviceability is an important factor. Conventional multi-stage pumps comprise a plurality of stages sequentially arranged so that the discharge portion of one stage feeds liquid into the suction portion of the next stage and each impeller is driven by a common impeller drive shaft. Rotation of the impeller drive shaft turns each impeller to force fluid outwardly into an internal passage which directs the fluid to the subsequent adjacent pump stage. However, these internal passages are difficult to clean and the pump must be substantially dismantled to permit cleaning. Predictably, these multi-stage pumps are used in applications where fouling or clogging is not of concern, such as well or water pumps, and these pumps are not conducive to use in mixed-media flow.
Additional improvements in pump characteristics, such as discharge head, would be advantageous in many applications. For example, in the above-noted sewage handling application, lift stations are expensive to build, with a cost that typically ranges between about forty five thousand dollars and several hundred thousand dollars and may even exceed a million dollars in some instances. A higher head, solids-handling, self-priming, centrifugal pump could be used to reduce the number of lift stations required to transmit wastewater to a wastewater treatment facility. Use of larger, higher-head trash pumps is possible, but such large pumps would have to operate at speeds higher than is generally advisable for a trash-type impeller, particularly in view of the fact that sewage pumps are expected to provide efficient operation for long periods of time without the need for frequent maintenance. Addition of pumps in series with existing pumps in a conventional manner is cumbersome or highly impractical given the space constraints imposed by the limited space available in conventional lift stations, and would be a costly proposition when the additional space requirements are factored into the designs of new, more expansive facilities.